There are a number of things wrong with this question and the diagram. One is the word "capacity". Vectors have a capacity. Deques don't. Vectors and deques have a size, the number of elements currently in the container. Capacity is something different. I'm assuming that size was meant here: There are four items in the deque prior to those operations.

The second thing wrong is those three holes in the middle of the diagram. As you noted, that should be two holes. Those two pop_front() calls will remove the first two items from the deque, and since all operations up until then were on the back, the removed items are from that initial content of four items.

The third thing that's wrong: What happened to the 1 and the 2?

Finally, the order of the first three items is incorrect. It should be 7,6,5 rather than 5,6,7.